1. Field of the Invention
The present invention relates to ceramic tubes and methods of forming same, and more particularly to ceramic arc tubes used in ceramic metal halide lamps.
2. Discussion of the Art
Ceramic arc tubes used for discharge lamp chambers were developed to operate at high temperatures on the order of 950° C. and higher. These types of lamps exhibit improved color temperature, color rendering, and luminous efficacies. Typically, ceramic discharge chambers are constructed from a number of individual components that are extruded or die-pressed from a ceramic powder. Prior practice employed a five component construction that included a central hollow cylinder substantially closed at either end by first and second plugs to which first and second legs were joined to the end plugs.
More recent developments have been directed to minimizing the number of joints, i.e., reducing the number of individual components, to establish an improved sealed arc chamber. For example, U.S. Pat. No. 6,004,503 discloses a method of making a ceramic arc tube for a metal halide lamp comprised of two components, i.e., a hollow body and an end cap. The '503 patent describes a two-part arc tube produced with a hollow body having an open end with a diameter approximately three to six percent (3-6%) greater than the opposite, closed end. That is, the hollow tube is tapered along its length and the patent disclosure is void of any description regarding the hermetic seal formed between the hollow body and the end cap. In the past, these components have been extruded or pressed and subsequently heated or fired to integrally sinter and join the components together.
Current injection molding practice for molding hollow body parts or cylindrical components employs a taper on a mold pin to aid in removing the part after molding. The degree of taper ranges from about one-half percent to about six percent (0.5%-6%) along the length of the pin. While this may be acceptable for many molded assemblies, the tapered conformation presents problems where a tapered end of one component joins a second component having a non-tapered surface. In attempting to monolithically join the components together, e.g., sealing or bonding the hollow body to the end cap, to form a hermetically sealed ceramic arc discharge tube, the mating of the two non-parallel surfaces has been determined to be very problematic. This is because of potential seal voids forming at the joined interface and precluding a hermetic seal. Thus, improving manufacturing steps, components, and addressing these needs will lead to longer-life lamps having improved monolithic seals between the hollow body and end cap.